Switching power converters offer higher efficiency as compared to linear regulators. Although linear regulators are relatively inexpensive, they regulate a lower output voltage from a higher input voltage by simply burning the difference as heat. As a result, a linear regulator typically burns more power than is actually supplied to the load. In contrast, a switching power converter regulates its output voltage by delivering relatively small increments of energy through the cycling of a power switch. The power switch in a switch-mode device is either off or on such that efficiency is markedly improved as compared to linear regulators. However, a power switch transistor does dissipate energy as it transitions from off to on and from on to off. This energy dissipation is proportional to the current and voltage being switching through the transistor. In addition, large rates of change for voltage and current through the power switch stress the device and cause significant electromagnetic interference (EMI).
To reduce the switching losses, device stress, and EMI, it is conventional to exploit the resonant voltage ringing that occurs across the power switch transistor when it is cycled off. The resonant voltage ringing causes the switch voltage to cycle through local minimums that are denoted as voltage valleys. A switching scheme that switches on the power switch at these local minimums is thus denoted as a valley-mode switching scheme. The resulting voltage waveforms for an example switching power converter configured to implement valley-mode switching are shown in FIG. 1A through 1C. FIG. 1A illustrates the on and off periods for a power switch S1, which is cycled on at a time t1 and cycled off at a time t2. The corresponding drain voltage for power switch S1 is shown in FIG. 1B. When the power switch S1 is switched on at time t1, the drain voltage is grounded but rebounds high at time t2 when the power switch is again switched off. The corresponding secondary winding current is shown in FIG. 1C. At time t2, the secondary current goes from zero to a maximum value. The stored energy is then delivered to the load as the secondary current ramps down to zero at a subsequent transformer reset time (trst). At this point, the resonant oscillations begin on the drain voltage for the power switch S1. The resonant oscillation has local minimums at times t3, t4, t5, and t6. The corresponding controller for power switch S1 then selects one of these minimums for the subsequent switch on time. For example, power switch S1 may again be cycled on at time t6 since this time is also a valley minimum.
In a control loop having a relatively constant pulse repetition frequency, the controller would tend to turn on the power switch S1 at substantially the same rate in each switching cycle. The result is that the EMI switching noise is concentrated at the principle switching frequency 200 and its harmonics 201 and 202 as shown in FIG. 2. To reduce the magnitude of the EMI at these peaks, it is thus conventional to dither the valley mode switching. For example, suppose that the controller's desired pulse on time falls between the valley minimums at times t4 and t5 in FIG. 1B. A controller with frequency dithering would then skip valleys and turn on power switch S1 at the subsequent valley at time t6. This valley skipping would be performed on a random basis so that the EMI noise is spread across the frequency spectrum as shown for dithered spectrum 205 in FIG. 2. Although such dithering is effective with regard to lowering the peak EMI magnitudes, there are applications such as capacitive sensing in the touch screens of smartphones and tablet computers that require very low noise emissions in certain sections of the frequency spectrum.
Accordingly, there is a need in the art for improved valley mode switching techniques with reduced EMI peak amplitudes while retaining frequency bands with virtually no EMI.